# Bypass start button on bladeless fan to make fan start when power on

I have a fan that I want it to turn on immediately when power on.

The fan is currently wired through an on/off switch which also included a switch for a motor that operated the oscillation of the fan. I need the fan to turn on when people come in without controlling, so I plan to connect it with an IR sensor switch. But first I need to skip the start button that currently use to on\off the fan.

I have tried to connect voltage and speed pin together but it doesn't work. (I did not remove the controlling board when I do that).

What is the correct way to bypass the fan switch for direct power on?

Here are photos of the front and back of the switch board.

I have hope for before posting this question because I read this: Bypass button press on fan for direct power on when plugged in Could I do something similar? I have tried to connect some pins together as mentioned above but it didn't work, what else can I try?

Update: Following Andrea suggestion, I checked and as he predicted, If I press start\stop button, it will short GND and ST together. So I include my drawing following Andrea answer so everyone can see it clearly and to check if it is the right way to solve this issue :

• What is the switch labelled SW for ? Can you add a (circuit) diagram which traces the connections to and from the START button. It looks like that button pulls a line low when pressed, while a resistor pulls that line high at other times. Is the start button also functioning as the stop button when pressed once more ?
– AJN
Jun 10 at 7:31
• hi AJN, I think SW is for Swing which means the fan will be oscillating when you press it. Yes, if you press START button once it will turn the fan on and press one more it will stop the fan. For speed button, it is by turning the button in the middle to make it high or low speed. Jun 10 at 9:10
• "I need the fan to turn on when people come in without controlling," - When and how do want it to turn off? Jun 21 at 5:37
• @BruceAbbott : when people go out and close the door Jun 22 at 12:27
• So you will turn the fan on with an IR sensor switch, but turn it off only when the door is closed (and the IR doesn't detect someone in the room?). How does closing the door turn the fan off? Jun 22 at 19:51

This is easy

1. Buy a fan that will turn on when you apply power. Forget all this digital control nonsense. Keep it simple

2. Buy any standard motion sensing light switch from the local hardware store

3. Wire them up

Done

Seriously, hacking into this is gonna be an exercise in frustration

• I also thinking of these, but seems it is harder to find your list, do you have some links ? Jun 10 at 9:09
• This answer provides no benefit to the person asking or anyone who reads it. Jun 10 at 9:25
• With arduino IR libraries that people already have created, could knock this out in a single day. Or with the relay option I mentioned in under 30 minutes. You on the other hand think "don't be self sufficient don't learn just throw money at a corporation" is the best solution. At best that is a comment not an answer. Jun 10 at 21:37
• @Passerby "Don't be self sufficient, don't learn, just throw money"??? WTH Have you ever heard of opportunity cost??? Yes, it's a concept that isn't included in engineering classes, just economics. YOU might be able knock this out in a day, but YOU aren't the one who is doing it ... OP is ... How many hours, and how much is his time worth? WAY WAY WAY more than a few new fans. BTW, are you aware both the impedance and bounce characteristics of a rubber/silicon switch are vastly different from a relay? You can't know the fans software will recognize a relay correctly. Jun 14 at 17:00
• But please do help this guy out if he wants to go that way. Just don't get him started and then abandon him. He clearly needs somebody to hold his hand all the way through. Jun 14 at 17:01

Since it is a soft switch, the ways of handling this is using a relay (preferred) or a transistor/mosfet (involves some more details of how the switch is wired), and either a one shot timer with initial delay on power on, or an easier solution involving a common microcontroller.

The microcontroller can either interface with a relay or mosfet/transistor or since the fan has an IR receiver, you could control it without internal modification. Put a microcontroller with IR diode on the same power circuit as the fan, and have it send a power on signal a second after it gets power. Plenty of examples online of how to control a IR fan with an arduino online.

• that sounds interesting, how do I have microcontroller with IR diode to send the righ signal to the IR receiver ? Jun 10 at 9:32
• @user1314404 something like this create.arduino.cc/projecthub/akarsh98/… Jun 10 at 21:40

The objective is to control the start/stop of this fan from a remote switch, activated by presence of people in the room and/or passing through a door.

Note: I think that the underlying spec is "presence of people", but it is said as "people getting in/out", "door open/closed". If we have 2 persons getting in and 1 getting out, the 1 remaining will have the fan switched off in the latter case. But "presence" is difficult to detect because any sensor will flicker when people are moving, so needs much more logic. Let's go ahead with in/out+door that is manageable.

The fan is activated by a push button that has 1 rest position and 1 momentary closed. Somewhere inside there is a circuit "counting" the pulses, first on, second off, third on, and so on. We must replicate this behavior with the external sensor+switch.

• To check how the push button is connected inside the fan: I see a "ST" wire, so it will be shorted to GND or VCC, and read by some logic inside (let's call it a "micro"). To check with multimeter if it is ST+GND or ST+VCC (or in case it is snot ST which wire).

Now the external switch must replicate this. The internal push button will be removed and two wires connected and driven as needed. We need a pulse (duration the time you take to press the original button), but I think it is the rising (or falling) edge that is sensed: so, pulse GND-VCC-GND (or opposedly VCC-GD-VCC, as said, to check) of limited duration (a fraction of second, or so).

The IR switch will close when someone is passing and will last for some time, not clear how long, especially if someone stands in the door, moves slowly, etc. If the IR switch sends a pulse every time the fan will start and stop too frequently: I suggests a led or light to warn not to hang around at the door.

The IR switch will receive two wires, ST and VCC (or GND). When it activates it must last for T seconds, with T possibly in the range 0.1-1. What happens is that the IR switch could switch for less time if the person is walking fast (ti-tlick, gone). So we need to capture short pulses from the IR switch (e.g. walking/running 1-2 m/s, the barrier (10 cm) is traversed in less than 5-10 ms).

This is the job for an "monostable multivibrator" such as the 4047 device: you get pulse in non shorter than e.g. 1 ms, and you get an output pulse guaranteed for at least 1 s.

• we want monostable: so pin 4 (!ASTABLE) to VCC and pin 5 (ASTABLE) to GND
• R1 & C1 calculated for output pulse duration: tout=1/(8.8R1C1) so about R1=100kohm and C1=1uF (even R1=1Mohm and C1=100nF will work, so C1=C2 below)
• trigger is our input signal from IR switch, to dampen with an input RC low-pass filter; R2 and C2 can be assigned to make 0.5 ms time constant, so R2=4.99kohm and C2=100nF
• for termination of unused inputs see datasheet

Now you should find place inside the fan and you run 2 wires from the IR switch. If you can find a DIP IC 4047, you can glue it upside down on the PCB and the few needed components can be placed there as well. (no SMDs please :) )

Last minute note: a push button somewhere in parallel to the IR switch may be advisable to bring the fan in a known state, just in case something messed up.

• Many thanks for the details and thoughtful solutions. Just to clarify some points you mentioned: basically it is the very small store room so I will assume there will be only one person at a time. But yes, it is correct to say the main requirement would be on\off when door open\close. Back to your solution, so can you draw on my pictures what can I try from there ? Any other pictures you want me to take ? Although I could capture your idea to certain level but to make sure it is correct, I would love to see it illustration on my photos. Thank you for the great help ! Jun 27 at 13:12
• Simple version: 1) In your photo you have the white connector where I see "ST" and then GnD and VCC. Check there the logic, if the push button shorts ST to GND or to VCC, ad if it is ST the right wire! Detached the connector you check with a multimeter if there is a momentary short and where when you press the button. 2) You need to locate an IR switch (or a microswitch for the door), but all of them will accept two wires. These wires are the two you identified at step 1). -- finished, but the IR switch can "mis-switch", so the use of the 4047. Jun 27 at 15:17
• The implementation is straightforward : once you identify the two wires going to the board for the push button, you branch one (the GND or VCC you got at step 1) and you extract (or cut) the ST wire. I said "branch" because you cannot remove the GND or VCC from the board, it is for other uses too. The ST wire instead is only for the push button and you will connect a long way to the IR switch, together with the branched GND or VCC with a similar wire. -- for long wires, run them twisted and put at least a capacitor (100nF) across them (for interference, but the fan should be protected inside). Jun 27 at 15:22
• @user1314404 What happened to this question? selected answer & bounty? Jun 29 at 6:36
• I am planing to try this again following your instruction and find enough components then I will give the bounty point to your answer Jul 1 at 5:39